Associative memory for accomplishing longest coincidence data detection by two comparing operations

ABSTRACT

An associative memory includes an array of CAM cells each having a transistor indicating a result from comparing a stored bit with a reference bit. The transistors are connected in serial in each row of the array to form a NAND circuit producing a signal representing that the bits stored in all the cells in the row are consistent with the reference bits or otherwise that the cell at the LSB of the row stores a bit consistent with a reference bit. Each column of the array includes a logic circuit for masking the bits except those continuous from the MSB toward the LSB which correspond to the bits of a word having the most bits continuous from the MSB toward the LSB and consistent with the bits of the reference word among the stored words. The NAND circuit in a row of the cells storing a word having the most bits continuous from the MSB toward the LSB and consistent with the corresponding bits of a reference word develops a signal representing the longest coincidence data detected.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an associative memory, moreparticularly to an associative memory for developing the address of astorage location storing therein data detected by the longestcoincidence data detection principle.

2. Description of the Background Art

As is known in the art, the associative memory, or often called CAM(Content Addressable Memory), is adapted to include storage locationsand develop data representative of the address of one of the storagelocations which contains data coincident with input or reference dataentered. The associative memory is advantageously applicable tosearching for routing information in telecommunications network systems,for example.

With the longest coincidence data detection system, the associativememory is adapted to receive input data and produce output datarepresentative of the address of a storage location which stores thereindata of which the entire bits are coincident with the corresponding bitsof the input data or otherwise data which have the most bits continuousfrom the MSB (Most Significant Bit) position toward the LSB (LeastSignificant Bit) position are coincident with the corresponding bits ofthe input data. The CAM memory is generally provided with the measuresof masking a specific bit or bits of a reference word of data to excludethe bit or bits from the coincidence detection.

An example of the longest coincidence data detection system is disclosedin Japanese patent laid-open publication No. 7782/1999, equivalent toU.S. Pat. No. 6,098,147 to Mizuhara. The apparatus for detecting thelongest coincidence data disclosed in the U. S. patent compares a wordof data stored in the associative memory and coincident with a referenceword of data with bits from the LSB position thereof masked beingincreased on the bit-by-bit basis by an incrementing device such as acounter to determine which bit position or positions continuous from theMSB of the stored word of data is or are coincident with a correspondingbit or bits of the reference word of data.

The system of increasing the mask bits one by one to detect the longestcoincidence data requires an extensive period of time until acoincidence is found out. For example, an application which is popularto the telecommunications network systems and in which an address valuerepresented by 32 bits is input to an associative memory as a referenceword of data would require to repeat comparison operations 32 times atthe worst. As taught by the U.S. patent, the CAM unit is divided intofour subsections, for example, each including eight bit positions, orone byte length, requiring the comparison operations to be repeatedeight times.

In an application of the CAM system structured as mentioned above tosearching for routing information in the network systems, the extensiveperiod of time required for finding coincidence would result in areduced throughput, thus causing the efficiency of the network tounfavorably be decreased.

The comparison operation of the CAM unit suffers from charging anddischarging the strayed capacitance caused by a lot of lengthy wiring.The conventional comparison operations continuously and repetitivelyaccomplished in the CAM unit would consume much more electric power.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improvedassociative memory.

More specifically, it is another object of the present invention toprovide an associative memory for accomplishing the longest coincidencedata detection in a minimum period of time.

Further, it is another object of the present invention to provide anassociative memory advantageously applicable to telecommunicationsnetwork systems.

It is still another object of the present invention to provide anassociative memory with its power consumption minimized.

In accordance with the present invention, an associative memorycomprises: an array of CAM cells each for storing therein a bit of datafed on a bit line, said array being formed in rows corresponding towords of the data and columns corresponding to bits of the word; each ofsaid CAM cells comprising a first transistor circuit taking either oneof a first and a second state, the first and the second staterepresenting that the bit stored in said CAM cell is consistent andinconsistent, respectively, with a bit of a reference word fed on thebit line; a first plurality of logic circuits provided correspondinglyto the rows, each of said first plurality of logic circuits producing afirst signal representing that the first transistor circuits of all ofthe CAM cells in corresponding one of the rows take the first state, andotherwise which of the first and second states the first transistorcircuit of the CAM cell at an LSB position of the corresponding one rowtakes; a second plurality of logic circuits provided correspondingly tothe columns, each of said second plurality of logic circuits detectingwhether or not all of the first transistor circuits in corresponding oneof the columns take the first state, and producing a second signal whenall of the first transistor circuits in the corresponding one columntake the first state; and a plurality of drive circuits providedcorrespondingly to the columns for each receiving a bit of an input orreference word, and driving the bit line of said CAM cells incorresponding one of the columns in response to the bit received; eachof said drive circuits being operative in response to the second signalproduced from corresponding one of said second plurality of logiccircuits to mask the bit line to cause the first transistor circuits ofall of the CAM cells in the corresponding one column to take the firststate; whereby the first signal is developed from the first logiccircuit in one of the rows which includes the first transistor circuitsall of which take the first state to thereby accomplish a longestcoincidence data detection.

More specifically, an associative memory includes an array of CAM cellseach having a transistor indicating a result from comparing a stored bitwith a reference bit. The transistors are connected in serial in eachrow of the array to form a NAND circuit producing a signal representingthat the bits stored in all the cells in the row are consistent with thereference bits or otherwise that the cell at the LSB of the row stores abit consistent with a reference bit. Each column of the array includes alogic circuit for masking the bits except those continuous from the MSBtoward the LSB which correspond to the bits of a word having the mostbits continuous from the MSB toward the LSB and consistent with the bitsof the reference word among the stored words. The NAND circuit in a rowof the cells storing a word having the most bits continuous from the MSBtoward the LSB and consistent with the corresponding bits of a referenceword develops a signal representing the longest coincidence datadetected.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the present invention will become moreapparent from consideration of the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 is a schematic circuit diagram showing part of a preferredembodiment of an associative memory in accordance with the presentinvention;

FIG. 2 shows in a schematic circuit diagram one CAM cell of theassociative memory shown in FIG. 1;

FIG. 3 is a schematic circuit diagram, similar to FIG. 2, showing oneCAM cell of an associative memory in accordance with an alternativeembodiment of the present invention;

FIG. 4 is a schematic circuit diagram, similar to FIG. 1, showing partof the associative memory in accordance with the alternative embodiment;

FIG. 5 schematically shows how words of data stored in the associativememory are coincident with an input reference word of data in theillustrative embodiments;

FIG. 6 is a chart useful for understanding how logical operations aremade on words of data stored in the associative memory with respect toan input reference word of data and OR operations are made on each bitposition between the stored words in the illustrative embodiments; and

FIG. 7 is a chart useful for understanding how the longest coincidencedata detection is performed in the illustrative embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, with reference to FIG. 2, one of CAM (Content Addressable Memory)cells of an associative memory in accordance with a preferred embodimentof the present invention, generally denoted with a reference numeral 10,includes an SRAM (Static Random Access Memory) circuit comprising acouple of NMOS (N type Metal Oxide Semiconductor) transistors NM21 andNM22 and a couple of inverters NV21 and NV22 interconnected to a pair ofbit lines 21 a and 21 b complementary to each other, as illustrated. TheNMOS transistors NM21 and NM22 have gate electrodes thereofinterconnected to a word line 20 as shown. The SRAM circuit is thusformed between the complementary bit lines 21 a and 21 b and the wordline 20 crossing the former, and functions as a storage unit for storingone bit of data. The SRAM circuit is further interconnected asillustrated to NMOS transistors NM15, NM23 and NM24, which function as acomparison circuit for comparing a bit of data stored with a inputreference bit of data. The NMOS transistor NM15 has its conductive pathor source-drain path inserted into a match line 12, as depicted.Although the illustrative embodiment is of N type MOS transistors, theinvention may of course be implemented by the opposite conductivity typeof MOS transistors.

Now, with reference to FIG. 1, the associative memory in accordance withthe preferable embodiment of the invention includes an array of threerows of CAM cells 10, each of the rows consisting of three CAM cells 10,just for the purpose of describing the invention. The CAM cells 10 ineach row are arranged in horizontal and have the word line 20 thereofinterconnected to each other to correspond to one word of data. Three ofthose rows constitute the associative memory with the pairs of bit lines21 a and 21 b interconnected to each other in vertical. With theillustrative embodiment, the memory can thus store three words of data,each of the words including three bits. The illustrative embodiment isdescribed merely for the purpose of understanding the invention, and maybe generally extended to an array of m x n CAM cells 10, where m and nare natural numbers, such as 32 and 512, respectively. In theembodiment, bits 1 and 3 are the MSB and LSB, respectively.

More specifically, the row of CAM cells 10 corresponding to word 1, forexample, have the NMOS transistors NM15 thereof connected in seriesacross the cells 10 as illustrated. In FIG. 1, those NMOS transistorsNM15 are denoted with reference codes NM15 with two digits of numeralsadded which have the first and second digits designating the row andcolumn numbers, i.e. bit and word numbers, respectively. The series oftransistors NM1511, NM1512 and NM1513 has its one end 1210interconnected through an NMOS transistor NM111 to the referencevoltage, or ground, and its other end 1213 through a PMOS (P type MetalOxide Semiconductor) transistor PM111 to another reference voltage, toprovide a precharge voltage, thus forming a general, dynamic NAND gateas a whole. The node 1213, at the LSB position, is interconnected to aninverter NV111 to develop an inverted voltage, which will be used for autility circuit, not shown, as an output signal indicating the word l.The transistors NM111 and PM111 have gate electrodes thereof connectedin common to a precharge control signal line 14 as illustrated.

As can be understood from FIG. 1, the remaining rows of CAM cells 10,corresponding to words 2 and 3, are also structured in the same manneras the row corresponding to word 1. The control signal line 14 isaccordingly connected to the gate electrodes of all of the prechargingtransistors NM111, NM112 and NM113 as well as PM111, PM112 and PM113. InFIG. 1, the associative memory includes drive circuits for driving theword lines 20, but is not shown just for simplicity.

Now, with respect to the column of CAM cells corresponding to bit 1, theNMOS transistors NM1511, NM1512 and NM1513 have the drain electrodes1211, 1212 and 1213 thereof, respectively, interconnected to input portsof a three-input NAND gate ND161, which has its output port 171interconnected to an input port of a bit line drive 131, as shown. TheNAND gate ND161 is adapted to perform NAND operation on its three inputs1211, 1221 and 1231. In other words, the NAND gate ND161 functions as anOR gate with respect to negative-logic inputs whereas it does as an ANDgate with respect to positive-logic inputs. In the specification,signals appearing in the circuitry are designated with the referencenumerals designating connections on which the signals appear. The NANDgate ND161 may be implemented by a static NAND circuit, generally a CMOScircuit, or a dynamic NAND circuit.

The bit line drive 131 has, in addition to the input port 171, threeother input ports, i.e. input data line 181, which is provided with aninput bit of data which is to be written into one CAM cell 10 or areference bit of data which is to be compared with a bit stored in oneCAM cell 10, a mask setting line 191, and a bit line drive controlsignal line 16. The bit line drive 131 also has its pair of output portsinterconnected to the pair of bit lines 21 a and 21 b, as illustrated.

As can be seen in FIG. 1, the associative memory includes additional bitline drives 132 and 133, which are provided correspondingly to bits orcolumns 2 and 3, and structured in the same manner as the bit line drive131 corresponding to bit 1. More specifically, on input data lines 182and 183, supplied are the remaining two input bits of data which are tobe written into two CAM cells 10 or two references bits of data whichare to be compared with bits stored in two CAM cells 10. The bit linedrives 132 and 133 have mask setting lines 192 and 193 connected,respectively. The bit line drive control signal line 16 is connected asshown in common to all of the bit line drives 131, 132 and 133.

The bit line drive 131 is adapted to be responsive to the control signal16 to bring the pair of bit lines 21 a and 21 b to the high levelthereof. The bit line drive 131 is also adapted to be responsive to thecontrol signal 16 to make the logical state of the pair of bit lines 21a and 21 b follow that of the input bit 181. In addition, the bit linedrive 131 is adapted to receive the mask setting signal 191 to renderits output bit lines 21 a and 21 b high when its input 171 from theassociated NAND gate ND163 is high. The remaining bit line drives 132and 133 are structured in the same manner as the drive 131.

More in general, with reference to FIG. 5, the associative memory of theembodiment in accordance with the invention includes the CAM cells 10 inan array of n words, each of the words comprising a certain number ofbits from its LSB to MSB, where n is a natural number such as 32. Whenthe associative memory is provided with a reference word of data, thememory compares the reference word with each of the stored words 1, 2,3,...,n. In the figure, exemplified results from the comparison areshown in such a manner that the bits stored in the memory consistentwith the bits of the reference word are depicted with hatching while notin blank. The associative memory is adapted to find out, among thestored data of words 1 through n, to which extent of the bits areconsistent with the corresponding bits of the reference wordcontinuously from the MSB position thereof.

In the illustrated example, word 4 has the coincident bits which aremost continuously from its MSB position as designated by the capitalletter A among those n words stored. With the example, the memory isadapted to distinguish the extent A of the continuous coincidence bitsfrom the remaining portion of bits B. In the specification, theoperation mentioned above is referred to as the logical operation on aword basis. The associative memory, having thus distinguished the lengthof bits A from the length of bits B in the stored data, will set up maskbits corresponding to the length of bits B, and use those mask bits inthe following fashion to identify word 4 as the longest coincidence wordof data among the words of data stored in the memory. The latteroperation mentioned above is referred to as the OR operation betweencorresponding bits in the specification.

More specifically, with reference to FIG. 6, the illustrative embodimentof the associative memory is first operative in the logical operation ona word basis. The associative memory, upon fed with a reference word ofdata, compares the reference word with stored words of data betweencorresponding bits so that ones of the CAM cells 10 of each row whichhave the stored bits continuous from the MSB position of the word andcoincident with the corresponding bits of the reference word render thematch line 12 thereof to the low (L) level thereof with the remainingCAM cells 10 of each row rendering the match line 12 thereof to the high(H) level thereof. In addition, the CAM cells 10 of the LSB position ofthe rows develop the logical state thereof on the output ports of theassociative memory. With the illustrative embodiment in FIG. 1, theinverters NV111, NV112 and NV113 produce the inverted state of the CAMcells 10 of the LSB position of the rows. As an example, in FIG. 5, thebits of the stored words coincident with the corresponding bits of thereference word input are depicted with the hatching, and in FIG. 6, thelogical levels H and L of the match lines 12 are shown with the mostcoincidence bits continuous from the MSB position hatched with the sameexample as FIG. 5.

The associative memory will then proceed to the OR operation betweencorresponding bits. The logical states of all the match lines 12 of eachcolumn or bit position are ORed with respect of negative-logic inputs,so that among the columns of the associative memory the OR operationcauses a column or columns in which any match lines 12 present the lowlevel thereof to produce the low-level output thereof. This correspondsto the AND operation with respect to positive-logic inputs, i.e. amongthe columns of the memory only a column or columns in which the entirematch lines 12 present the high level thereof produce or produces thehigh-level output thereof. Consequently, it can be said that the ORoperation between corresponding bits determines the extent of ones ofthe columns of the memory which correspond to the most coincidence bitscontinuous from the MSB position. With the specific example shown inFIG. 6, resultant information representing the extent A or B,corresponding to word 4, will be obtained in its inverted form, asillustrated in the top part of the figure. In the illustrativeembodiment shown in FIG. 1, the OR operation between corresponding bitsis implemented by the NAND gates ND161, ND162 and ND163.

More specifically, in operation, the CAM cell 10 works in the samemanner as a conventional SRAM so far as the writing operation of a bitof input data is concerned. For example, the CAM cell 10, FIG. 2,operates in its positive logic so that one 21 a of the complementary bitlines 21 a and 21 b is the positive phase. In order to write a binary“1” into the CAM cell 10, the bit lines 21 a and 22 b are placed to thehigh and low levels thereof, respectively, by the associated bit linedrive, e.g. 131, with the word line 20 kept in its high level, resultingin the transistors NM21 and NM 22 conductive. This causes the potentialof the gate electrode 14 of one of the storage transistor NM23 to beraised. By contrast, in order to write in a binary “0” into the CAM cell10, the bit lines 21 a and 22 b are rendered low and high, respectively.This causes the potential of the gate electrode 16 of the other storagetransistor NM24 to be raised. Although the illustrative embodiment isdirected to positive logic, the invention may of course be applicable tonegative logic devices.

In the searching or reference mode of the CAM cell 10, the word line 20is rendered to its low level to cause the transistors NM21 and NM22 tobe non-conductive. A bit of reference data is then applied from theassociated bit line drive, e.g. 131, to the complementary bit lines 21 aand 21 b in such a fashion that the bit lines 21 a and 21 b take thehigh and low levels thereof, respectively, representing a binary “1”,and vice versa. For example, a binary “1” is stored in the CAM cell 10so that one of the transistors NM23 is maintained conductive. Under thecircumstances, if a binary “1” is applied to the cell 10 as a referencebit, i.e. the bit line 21 a is rendered high, then the transistor NM15goes to its conductive state. If a binary “0” is applied to the cell 10,the transistor NM15 becomes non-conductive. By contrast, a binary “0” isstored in the CAM cell 10 so that the other transistor NM24 ismaintained conductive. If a binary “0” is applied to the cell 10 as areference bit, i.e. the bit line 21 b is rendered high, then thetransistor NM15 goes to its conductive state. If a binary “1” is appliedto the cell 10, the transistor NM15 becomes non-conductive. This impliesthat when the bit stored in the CAM cell 10 is consistent with the bitof a reference word supplied the transistor NM15 is rendered conductive,and otherwise non-conductive.

In order to mask a bit position during comparison operation, the coupleof bit lines 21 a and 21 b of a CAM cell 10 of that bit position areboth brought to the high level thereof by the associated bit line drive131, for example. This causes the transistor NM15 to be conductiveirrespective of the conductive state of the storage transistors NM23 andNM24, i.e. the stored bit of the CAM cell 10, thus resulting in theconductive state of the transistor NM15. This logical state of the CAMcell 10 is equivalent to that of the coincidence occurring on that bitso as to be excluded from comparison with a reference bit, that is,masked.

Now, returning to FIG. 1, the bit line drives 131, 132 and 133 areresponsive to the control signal 16 generated by a control circuit, notshown, to render the pair of bit lines 21 a and 21 b to the high levelthereof to cause the NMOS transistors NM1511-NM1513, NM1521-NM1523 andNM1531-NM1533 conductive, respectively. Then, the precharge signal 14goes to its low level by the not-illustrated control circuit, givingrise to the PMOS transistors PM111, PM112 and PM113 conductive and theNMOS transistors NM111, NM112 and NM113 non-conductive. This causes thenodes 1210-1213, 1220-1223 and 1230-1233 to be rendered high, so thatthe NAND gates ND161, ND162 and ND163 produce the low levels on theoutputs 171, 172 and 173, respectively.

The bit line drives 131, 132 and 133 are then fed with the controlsignal 16 so as to receive the respective bits of the reference word.Thereafter, the precharge signal 14 is returned to its high level torender the PMOS transistors PM111, PM112 and PM113 non-conductive andthe NMOS transistors NM111, NM112 and NM113 conductive. For example, ifthe bit line drive 133 receives its mask setting signal 193, then thedrive 133 brings its output bit lines 21 a and 21 b high.

As described earlier, the transistor NM15 of a CAM cell 10 becomesconductive when that cell 10 stores therein a bit consistent with acorresponding bit of a reference word input. For example, when acoincidence occurs on the bits 1, 2 and 3 of the word 1 in the array ofCAM cells 10 shown in FIG. 1, the node 1213 of the LSB position of theword 1 becomes low to cause the inverter NV111 to develop its high levelon its output 111. As understood from this situation, only whenever allof the bits of a word stored is consistent with those of a referenceword, the output from the inverter such as NV111 of that word will berendered high.

Referring now to FIG. 7, a three-bit reference word of data is appliedto the array of CAM cells 10 shown in FIG. 1, so that coincidences occurjust with the bits 1 and 2 of the word 1, the bit 1 of the word 2, andthe bits 2 and 3 of the word 3, as depicted with the hatching in FIG.7.Under the circumstances, all the inverters NV111, NV112 and NV113produce the low-level outputs 111, 112 and 113, respectively. In thisstage of comparing operation, if any of the rows 1, 2 and 3 outputs apositive or high level on its corresponding output 111, 112 or 113,namely, the word stored in the row is fully consistent with thereference word entered, then the associative memory may finish thecomparison operation without proceeding to the second comparingoperation described later.

Under the instant situation, the transistors NM1511 and NM1512 of therow or word 1 are conductive and the remaining one NM1513 isnon-conductive so that the nodes 1211 and 1212, associated with theextent from the MSB bit to the second bit, are low, while the node 1213,the remaining bit, is high, as depicted in FIG. 7. Similarly, thetransistor NM1521 of the row or word 2 is conductive and the remainingones NM1522 and NM1523 is non-conductive so that the node 1221,associated with the MSB bit, is low and the nodes 1222 and 1223,associated with the remaining bits, are high. Further, the transistorsNM1532 and NM1533 of the row or word 3 are conductive and the remainingone NM1531 is non-conductive so that all the nodes 1231, 1532 and 1533,extending from the MSB bit to the LSB bit, are maintained high.

As seen from FIG. 7, the dynamic NAND gate, consisting of the threetransistors NM 1511, NM1512 and NM1513, connected in serial, of the rowor word 1 receives as its three inputs the low level in the extentcontinuous from the MSB position to the second bit position coincidentwith the corresponding bits of the reference word, and the high level inthe extent of the remaining bit to the LSB position, the LSB per se inthe example. Further, the logical state, low in the example, of thetransistor NM1513, corresponding to the LSB bit, is inverted by theinverter NV111 to be output from the output port 111. The remainingdynamic NAND gates, consisting of the transistors NM 1521, NM1522,NM1523, and NM1531, NM1532, NM1533, connected in serial, of the rows orwords 2 and 3, respectively, achieve the same operation as that of therow 1.

The three-input NAND gate ND161 receives the logical state of the nodes1211, 1221 and 1231. With the exemplified case shown in FIG. 7, the NANDgate ND161 produces its high level from its port output 171 to the bitline drive 131 associated therewith. Similarly, the three-input NANDgate ND162 receives the low level of the node 1212 and the high level ofthe nodes 1222 and 1232, with the exemplified case, to produce its highoutput from its port 172 to the bit line drive 132. The remainingthree-input NAND gate ND163 receives the high level of the nodes 1213,1223 and 1233, with the exemplified case, to produce its low level fromits port 173 to the bit line drive 133.

It is important to note that the high level signals 171 and 172developed from the NAND gates ND161 and ND162, respectively, representthe extent of bit positions of the stored word which are continuouslycoincident from the MSB position with the corresponding bits of thereference word, resultantly from the OR operation made on the low stateof the nodes 1211, 1221, 1231, and 1212, 1222, 1232, respectively. Inother words, the outputs 171, 172 and 173 from the NAND gates ND161,ND162 and ND163, respectively, represent to which extent of the bits ofthe words stored in the associative memory are coincident continuouslyfrom the MSB position of the reference word. In the instant example, thehigh level developed from the NAND gates ND161 and ND162 represents thattwo continuous bit positions from the MSB involve coincidence due to thefact that word 1 has its MSB and second bits coincident with thecorresponding bits of the reference word. The outputs 171, 172 and 173are referred to as a longest coincidence signal in the specification.

With this exemplified case, only the NAND gate ND163 delivers its highlevel 173 to the associated bit line drive 133. The bit line drive 133in turn produces high-level signals on its pair of bit lines 21 a and 21b under the control of the control signal 16 provided. This means thatthe bit 3 is masked from the following comparison operation. Inresponse, the column 3, with this example, the transistors NM1513 andNM1523 become conductive in addition to the transistor NM1533 alreadyconductive. In the row 1, since the transistors NM111, NM1511 and NM1512are already made conductive, the rendering conductive of the transistorNM1513 now causes the inverter NV111 to produce its high level on itsoutput 111. On the other hand, the rows 2 and 3 still include thetransistors NM1522 and NM1531 non-conductive, respectively, to maintainthe nodes 1223 and 1233 in the high level thereof, thus developing thelow level on the outputs 112 and 113 from the inverters NV112 and NV113,respectively. The row or word 1, producing the high level output 111, isthus identified as the longest coincidence data with respect to thereference word input.

With the illustrative embodiment described above, the comparisonoperations are performed merely twice, namely, the first comparisonwithout using bit masking to determine which of the bit positions to bemasked and thereafter the second comparison with a mask bit or bits thusdetermined being used, irrespective of how many bits a reference wordincludes. It is to be noted that the associative memory may be adaptedto accomplish only the first comparing operation without advancing tothe second comparing operation in an application in which such a word ofdata stored in the memory which is completely coincident with areference word of data is to be searched for.

Now referring to FIG. 3, a CAM cell 10 a included in an alternativeembodiment of the invention comprises an additional PMOS transistorPM41, which has its gate, source and drain electrodes connected to thematch line 12, a reference voltage and a longest coincidence line 47,respectively.

In the embodiment shown in FIG. 4, the CAM cells 10 a are arranged in anarray of three rows and three columns for the purpose of describing theinvention only. In the specification, the like elements are denoted withthe same reference numerals, without repeating a redundant explanationthereon. In the array thus shown, the column 1 includes a longestcoincidence line 471, which interconnects in common the PMOS transistorsPM4111, PM4121 and PM4131. In the remaining columns 2 and 3, longestcoincidence lines 472 and 473 interconnect the PMOS transistors PM4112,PM4122, PM4132, and PM4113, PM4123, PM4133, respectively, in common.

The longest coincidence lines 471, 472 and 473 are connected on one handto an input port corresponding to the ports 171, 172 and 173 of the bitline drives 131, 132 and 133, respectively, shown in FIG. 1, and on theother hand to the ground, a reference voltage, through additional NMOStransistors NM461, NM462 and NM463, respectively. The NMOS transistorsNM461, NM462 and NM463 are for use in discharging the longestcoincidence lines 471, 472 and 473, respectively, and have the gateelectrodes interconnected in common to a discharge signal 15. In FIG. 4,the PMOS transistors PM41, FIG. 3, are denoted with the reference codePM41 with two digits of numerals added which have the first and seconddigits designating the row and column numbers, respectively. The longestcoincidence lines 47, FIG. 3, are denoted with the reference numeral 47also with one digit of numeral added designating the column number.

In operation, the CAM cell 10 a operates in the same manner as the CAMcell 10 in storing a bit and comparing a stored bit with a referencebit. The array of CAM cells 10 a also operates in the same manner as thearray of CAM cells 10. More specifically, the bit line drives 131, 132and 133 are controlled in response to the control signal 16 to renderthe pair of bit lines 21 a and 21 b to the high level thereof to causeall the NMOS transistors NM1511-NM1513, NM1521-NM1523 and NM1531-NM1533conductive, respectively. Then, the precharge signal 14 becomes low torender the PMOS transistors PM111, PM112 and PM113 conductive and theNMOS transistors NM111, NM112 and NM113 non-conductive, resulting in thehigh states of the nodes 1210-1213, 1220-1223 and 1230-1233. During thisperiod of operation, the high levels of the nodes 1211, 1212 and 1213render the transistors PM4111, PM4112 and PM4113 non-conductive,respectively, which have the drain thereof connected to the longestcoincidence lines 471, 472 and 473, respectively.

Following that, the high level of the discharge signal 15 is applied tothe gate electrode of the discharge NMOS transistors NM461, NM462 andNM463 by a control circuit, not shown, to conduct the source-drain paththereof to thereby render the longest coincidence lines 471, 472 and 473low. Thereafter, the discharge signal 15 returns to its low level sothat the NMOS transistors NM461, 462 and 463 go to the non-conductivestates thereof. Successively, the control signal 16 is applied to thebit line drives 131, 132 and 133 with the bits of reference word 181,182 and 183 applied thereto, respectively. The precharge signal 14 isthen returned to its high level to place the PMOS transistors PM111,PM112 and PM113, and the NMOS transistors NM111, NM112 and NM113 in thenonconductive and conductive states thereof, respectively.

In the case of the example described with reference to FIG. 7,coincidences occur just with the bits 1 and 2 of the word 1, the bit 1of the word 2, and the bits 2 and 3 of the word 3, so that all theinverters NV111, NV112 and NV113 produce the low-level outputs 111, 112and 113, respectively, as described before with reference to theembodiment shown in FIG. 1. That is also the case with the alternativeembodiment.

More specifically, the transistors NM1511 and NM1512 of the row 1 areconductive and the remainder NM1513 is non-conductive so that the nodes1211 and 1212 are low while the node 1213 is high. The transistor NM1521of the row 2 is conductive and the remaining ones NM1522 and NM1523 arenon-conductive so that the node 1221 is low and the nodes 1222 and 1223are high. The transistors NM1532 and NM1533 of the row 3 are conductiveand the remaining one NM1531 is nonconductive so that all the nodes1231, 1532 and 1533 are maintained high.

The conductive state of the PMOS transistors PM4111 and PM4121 bringsthe longest coincidence line 471 of the column or bit 1 high. Similarly,the conductive state of the PMOS transistor PM4112 brings the longestcoincidence line 472 of the column or bit 2 to its high level. Thelongest coincidence line 473 of the column or bit 1 is kept low sincethe PMOS transistors PM4113, PM4123 and PM4133 take the non-conductivestate thereof. As understood, the logical state of the longestcoincidence lines 471, 472 and 473 represent to which extent of the bitsof the words stored in the associative memory are coincidentcontinuously from the MSB position of the reference word. In otherwords, the lines 471, 472 and 473 produce signals equivalent to thelongest coincidence signals as described with reference to theembodiment shown in FIG. 1.

Only the longest coincidence line 473 develops its low level to theassociated bit line drive 133 with the instant example. The bit linedrive 133 in turn produces high-level signals on its pair of bit lines21 a and 21 b in response to the control signal 16 provided. The bit 3will thus be masked from the following comparison operation. Inresponse, the column 3, with this example, the transistors NM1513 andNM1523 become conductive in addition to the transistor NM1533 alreadyconductive. In the row 1, the transistors NM111, NM1511 and NM1512 arealready conductive, and the rendering conductive of the transistorNM1513 now causes the inverter NV111 to produce its high level on itsoutput 111. On the other hand, the lows 2 and 3 still include thenon-conductive transistors NM1522 and NM1531, respectively, to keep thenodes 1223 and 1233 in the high level thereof, thus developing the lowlevel on the outputs 112 and 113 from the inverters NV112 and NV113,respectively. The row or word 1 is thus identified as the longestcoincidence data with respect to the reference word input.

The alternative embodiment involves the same advantages described withreference to the embodiment shown in FIG.1. In addition, the associativememory including the CAM cells 10 a in accordance with the alternativeembodiment is smaller in size and more rapid in operational speed thanthe associative memory including the CAM cells 10 of the FIG. 1embodiment. Because, in order to produce the longest coincidencesignals, the associative memory including the CAM cells 10 a inaccordance with the alternative embodiment does not include the staticor dynamic NAND circuits ND161, ND162 and ND163 requiring wiringconnections provided for the entire bits of each row but merely thewiring connections for the longest coincidence lines 471, 472 and 473each provided in common to the entire rows and the associated MOSdevices. That advantage is more predominant in applications where alarger number of word storage locations are included.

In summary, in accordance with the present invention, the comparisonoperations are executed merely twice, i.e. the first operation isperformed without using bit masking to determine which of the bitpositions to be masked and thereafter the second operation with a maskbit or bits thus determined being used, irrespective of how many bits areference word includes. The associative memory therefore accomplishesthe longest coincidence data detection in a minimized period of timewith its power consumption minimized. The associative memory isadvantageously applicable to telecommunications network systems.

The entire disclosure of Japanese patent application No. 2000-8714 filedon Jan. 18, 2000, including the specification, claims, accompanyingdrawings and abstract of the disclosure is incorporated herein byreference in its entirety.

While the present invention has been described with reference to theparticular illustrative embodiments, it is not to be restricted by theembodiments. It is to be appreciated that those skilled in the art canchange or modify the embodiments without departing from the scope andspirit of the present invention.

What is claimed is:
 1. An associative memory comprising: an array of CAM(Content Addressable Memory) cells each for storing therein a bit ofdata fed on a bit line, said array being formed in rows corresponding towords of the data and columns corresponding to bits of the word; each ofsaid CAM cells comprising a first transistor circuit taking either oneof a first and a second state, the first and the second staterepresenting that the bit stored in said CAM cell is consistent andinconsistent, respectively, with a bit of a reference word fed on thebit line; a first plurality of logic circuits provided correspondinglyto the rows, each of said first plurality of logic circuits producing afirst signal representing that the first transistor circuits of all ofthe CAM cells in corresponding one of the rows take the first state, andotherwise which of the first and second states the first transistorcircuit of the CAM cell at an LSB (Least Significant Bit) position ofthe corresponding one row takes; a second plurality of logic circuitsprovided correspondingly to the columns, each of said second pluralityof logic circuits detecting whether or not all of the first transistorcircuits in corresponding one of the columns take the first state, andproducing a second signal when all of the first transistor circuits inthe corresponding one column take the first state; and a plurality ofdrive circuits provided correspondingly to the columns for eachreceiving a bit of an input or reference word, and driving the bit lineof said CAM cells in corresponding one of the columns in response to thebit received; each of said drive circuits being operative in response tothe second signal produced from corresponding one of said secondplurality of logic circuits to mask the bit line to cause the firsttransistor circuits of all of the CAM cells in the corresponding onecolumn to take the first state; whereby the first signal is developedfrom the first logic circuit in one of the rows which includes the firsttransistor circuits all of which take the first state to therebyaccomplish a longest coincidence data detection.
 2. The memory inaccordance with claim 1, wherein each of said first plurality of logiccircuits comprises conductive paths of said first transistor circuit ofthe CAM cells interconnected in serial in a direction of the row toproduce the first signal from the LSB position of the word; each of saidsecond plurality of logic circuits having input ports interconnected tothe conductive path of different ones of the first transistor circuitsacross the rows to produce the second signal.
 3. The memory inaccordance with claim 2, wherein said CAM cells comprise first MOS(Metal Oxide Semiconductor) transistors; said first transistor circuitcomprising a second MOS transistor having a source-drain pathinterconnected in serial to the second MOS transistor of adjacent onesof said CAM cells in the direction of the row, said first logic circuitforming a first AND circuit comprising said second MOS transistorsinterconnected in serial to produce the first signal; each of saidsecond plurality of logic circuits comprising a second AND circuithaving input ports interconnected to the source-drain path of differentones of the second MOS transistors across the rows for producing thesecond signal to corresponding one of said drive circuits when all ofthe source-drain paths of the second MOS transistors in correspondingone of the columns take the first state.
 4. The memory in accordancewith claim 1, wherein each of said first plurality of logic circuitscomprises conductive paths of said first transistor circuit of the CAMcells interconnected in serial in a direction of the row to produce thefirst signal from the LSB position of the word; each of said CAM cellscomprising a second transistor circuit having a control electrodeconnected to a conductive path of said first transistor circuit; saidsecond transistor circuits of the CAM cells in each of the columnshaving a conductive path interconnected in parallel across the rows toproduce the second signal to corresponding one of said drive circuits toform different one of said second plurality of logic circuits.
 5. Thememory in accordance with claim 4, wherein said CAM cells comprise firstMOS transistors; said first transistor circuit comprising a second MOStransistor having a source-drain path interconnected in serial to thesecond MOS transistor of adjacent ones of said CAM cells in thedirection of the row, said first logic circuit forming a first ANDcircuit comprising said second MOS transistors interconnected in serialto produce the first signal; said second transistor circuit comprising athird MOS transistor having a gate electrode connected to thesource-drain path of said first transistor circuit, said third MOStransistors in each of the columns having a source-drain path connectedin parallel across the rows to corresponding one of said drive circuits.6. The memory in accordance with claim 5, wherein each of said secondplurality of logic circuits further comprises a third transistor circuitprovided in corresponding one of the columns for discharging a parallelconnection of the source-drain paths of the third MOS transistors in thecorresponding one column.
 7. The memory in accordance with claim 3,wherein said first logic circuits are formed by dynamic NAND circuits.8. The memory in accordance with claim 3, wherein said second logiccircuits are formed by dynamic or static NAND circuits.
 9. The memory inaccordance with claim 3, wherein said first and second MOS transistorsare of N type.
 10. The memory in accordance with claim 5, wherein saidfirst and second MOS transistors are of N type, and said third MOStransistors are of P type.
 11. The memory in accordance with claim 6,wherein said first and second MOS transistors are of N type, and saidthird MOS transistors are of P type, said third transistor circuitscomprising NMOS transistors.